Currently, the trend in the electronics industry is toward producing electronic equipment containing surface mount components. Surface mount components are electronic devices, such as transistors, resistors, chip carriers and the like which are designed for mounting directly to metallized areas on a major surface of a printed circuit board, in contrast to "through hole" type components which have their leads inserted through and solder bonded to metal-plated apertures in the board. Presently, there are two types of surface mount components, those which are "leaded" and those which are "leadless." Leaded surface-mount components have leads which are directly connected to the metallized areas on the printed circuit board. Leadless surface mount components are connected to the metallized areas on the printed circuit board by metallized pads on the exterior of the case of the component.
Although careful efforts are made at screening defective surface mount components prior to their being soldered to a printed circuit board, nonetheless, it is not unusual for a defective component to escape such screening and be mounted on the circuit board. Then, to repair the circuit board, the defective component must be removed and a replacement component soldered in its place. In the past, soldering of either leaded or leadless replacement components to the board has been accomplished by using a syringe to apply fresh solder paste to the metallized pads on the circuit board formerly bonded to the defective component. Unfortunately, the solder paste applied by the syringe tends to be uneven, yielding solder joints of poor quality.
Alternatively, in the case of a leadless component, a controlled amount of solder can be applied to the pads on the component case to allow the component to be soldered in place of the one which is defective without the need to apply solder paste to the circuit board. A controlled amount of solder can be applied to the metallized pads on a replacement leadless surface mount component by first placing the component in a fixture in intimate contact with a baseplate having a plurality of cavities therein arranged in a pattern the same as that of the pads on the case of the component. The fixture aligns the leadless component such that each metallized pad is contiguous with a solder sphere seated in a separate one of the cavities. The solder spheres are then reflowed so each bonds to a separate one of the metallized pads on the component, causing each pad to have a mass or "bump" of solder depending therefrom. Following the reflow operation, the pads on the leadless component are said to have been "bumped".
During the process of bumping the pads on the leadless surface mount component, the surface tension of the reflowed solder spheres is sufficient to cause the pads to self-align with the solder spheres. The self-alignment between the pads and the solder spheres compensates for small positioning errors that may initially exist therebetween. However, if one were to try to bump the leads of a leaded device in the same manner as the pads on the leadless component, one would find the surface tension of the solder spheres to be insufficient to cause the tips of the leads to self-align with the solder spheres. Thus, no compensation would be provided for any initial positioning errors which may exist between the lead tips and the solder spheres. The tips of the leads of the leaded component are arcuate in shape and would tend to slide off the solder spheres when placed thereon, often giving rise to an initial positioning error between the lead tips and the solder spheres.
Therefore, there is a need for a technique for applying a controlled amount of solder to the tips of the leads of a leaded component which assures that lead tips remain aligned with the solder spheres during the reflow thereof.